As is well-known, band-shaped glass ribbons formed by an overflow down-draw method, a slot down-draw method, a float method, a redraw method, or the like are cut into a predetermined dimension and produced as glass substrates having a substantially rectangular shape. In reality, the glass substrates of this type have been used for producing a flat panel display (FPD) typified by a plasma display plate (PDP), a liquid crystal display (LCD), a field-emission display (FED), an organic light-emitting diode (OLED) display, or a solar cell, a secondary battery, or the like.
Under the circumstances, these FPDs and the like have been prompted to reduce in weight, and hence glass substrates to be used as main components of the FPDs and the like are being reduced in thickness. In particular, organic light-emitting diodes are being used not only for a display in which fine three primary colors are allowed to blink by TFTs as in an OLED display, but also for planar light sources such as light sources for an LCD backlight or indoor illumination by emitting light with only a simple color (for example, a white color). Then, in an illumination apparatus of organic light-emitting diodes, a light-emitting surface can be deformed freely if a glass substrate has flexibility. Therefore, the glass substrate to be used in the illumination apparatus is also being promoted to remarkably reduce in thickness from the viewpoint of ensuring sufficient flexibility.
As a method of producing a thin glass substrate to be used for an FPD, an illumination apparatus, or the like, a down-draw method typified by the above-mentioned overflow down-draw method, slot down-draw method, or redraw method is considered to be preferred. Of those down-draw methods, the overflow down-draw method and the slot down-draw method include a forming step of forming a glass ribbon by allowing molten glass to flow down into a band shape (plate shape) from a forming part and cooling both widthwise ends of the band-shaped molten glass with cooling rollers. The redraw method includes a forming step of forming a glass ribbon by softening a glass sheet by re-heating and allowing the softened glass sheet to descend. Then, any of the down-draw methods include an annealing step of removing an internal strain by annealing the glass ribbon inside an annealer while allowing the glass ribbon to descend after executing the forming step, and a cutting step of cutting the glass ribbon after executing the annealing step.
The down-draw method is described in detail. Patent Literature 1 discloses a configuration in which, when the annealing step is executed, a glass ribbon inside an annealer is stretched in a downward direction by tensile rollers provided in the lowermost stage of the annealer. Claim 1 of Patent Literature 1 recites that the thickness of a glass substrate is 0.7 mm or less. However, [0051] and [0053] of this literature respectively describe that the thickness of a widthwise center portion is 0.7 mm and the thickness of the widthwise center portion is 0.63 mm. Therefore, it is understood that the down-draw method described in this literature is directed to the formation of a glass ribbon having a thickness of about 0.6 to 0.7 mm.
Further, Patent Literature 2 discloses a configuration in which, when the annealing step is executed, a glass ribbon inside an annealer is stretched in a downward direction by all the tensile rollers provided in a plurality of stages (three stages) in the vertical direction in the annealer. [0004] and [0042] of this literature describe that the thickness of a glass ribbon (sheet glass) is 0.7 mm, and hence it is understood that the down-draw method described in this literature is directed to the formation of a glass ribbon having a thickness of about 0.7 mm.
Further, Patent Literature 3 and Patent Literature 4 disclose a configuration in which all the tensile rollers provided in a plurality of stages in the vertical direction in an annealer draw a glass ribbon in a downward direction, and all the tensile rollers provided in a plurality of stages in the vertical direction in a cooling chamber present below the annealer also draw the glass ribbon in a downward direction. Each literature does not describe the thickness of a glass substrate or a glass ribbon, but the detailed description of the invention in each literature, substantially, merely describes a glass substrate for a liquid crystal display. Therefore, considering the technical standard at a time of filing of the application, it is understood that the down-draw method described in each literature is also directed to the formation of a glass ribbon having a thickness of about 0.7 mm.
In addition, Patent Literature 5 discloses a configuration in which guide rollers are provided at an interval dimension larger than the thickness of both widthwise ends of a glass ribbon in a plurality of stages in the vertical direction in an annealer, and the glass ribbon is guided downwardly without being held by the guide rollers. The down-draw method described in this literature is directed to the formation of a glass ribbon (glass film ribbon) having a thickness of 500 μm or less, further, a glass ribbon having a thickness of 200 μm or less.